A couple of points on this. First the trigger voltage of Canon units is kept quite low to ensure that if the flashgun is slid in / out of the hotshoe with the gun switched on that it doesn't short through the communication terminals and potentially fry the circuits.

Second, hight trigger voltages, i.e. 250V and over can cause pitting of the flash contact points in the shutter due to arcing if used excessively. Occasional use is not a problem.

So if you are careful to mount / dismount the gun only with it switched off and if you are not using it too much then really you shouldn't have a problem.